1. Field of the Invention
The present invention relates generally to surface acoustic wave (SAW) transducers and, more specifically to a new family of surface acoustic wave single-phase unidirectional transducers (SPUDTs) by using uniform width, single-level electrodes lying on a uniform grid of a piezoelectric substrate where unidirectionality is achieved by selectively etching reflective structures into the substrate either in the spaces located between adjacent electrodes or under selected electrodes of the transducer. A SAW filter, resonator or delay line can also be formed by fabricating two such transducers on a single substrate, each having surface acoustic waves radiating towards the other.
2. Description of the Prior Art
Surface acoustic wave (SAW) devices are fabricated by delineating thin-film conductor patterns on piezoelectric single-crystal substrates. These devices make use of surface acoustic waves, Rayleigh waves, or like waves that propagate at the velocity of sound along a surface of the crystal. In a conventional SAW device, each wave is excited by an input electrical signal applied to a set of interdigitated electrode transducers that have been disposed on the surface of the crystal. The wave propagates along the surface of the crystal where it is detected by a separate set of interdigitated electrodes that are also disposed on the surface of the crystal. The shape and spacing of the electrodes determines the center frequency and the band shape of the detected wave. Generally, the smaller the width of the electrode or the lower the sampling rate (i.e. number of electrodes per period), the higher the operating frequency of the device. The amplitude of the propagated wave at a particular frequency is determined by the constructive interference between the waves generated at individual electrodes in the set of input interdigitated electrodes.
Single-phase unidirectional transducer (SPUDT) SAW filters are a distinct type of SAW device. Such filters are widely used in communication systems because of their small group delay ripple (GDR), low insertion loss, and simplicity of use in matching circuits. The basic principle of a SPUDT is to have SAW reflectors displaced inside the SAW transducer in such a way that the reflection and transduction centers of acoustic waves are shifted spatially in phase by plus or minus 45 degrees or plus or minus 135 degrees. Upon satisfaction of this principle, a transduction wave and a reflected wave constructively enhance each other in one direction and destructively cancel each other in other directions, thereby causing the waves to propagate unidirectionally through the transducer. A properly designed SPUDT filter can achieve low insertion loss and high triple transit suppression at the same time by using relatively simple RLC (resistor inductor capacitor) matching circuits at the input and output transducers. The triple transit is caused by regenerated SAW between transducers that cause large amplitude ripples and GDR in the passband.
Various types of SPUDTs have been developed since the 1980""s and most of them rely exclusively on the metal electrode fingers to act as acoustic wave reflectors. For example, distributed acoustic reflection transducer (DART) SPUDT techniques are widely used in the SAW industry for low loss, high performance SAW filter applications. And recently, group SPUDT (GSPUDT) and dithered SPUDT (DSPUDT) techniques have been developed for high frequency filter applications. And while there are advantages to each of these techniques, each has certain limitations.
For example, a typical DART SPUDT, like that described in the publication xe2x80x9cDesign of Low-loss SAW Filters Employing Distributed Acoustic Reflection Transducers,xe2x80x9d by Kodoma et al., IEEE Ultrasonics Symposium, 1986, has advantages that include single level metallization, strong electrical coupling and strong finger (electrode) reflectivity. But the small critical geometry of 0.6 xcexcm (finger width=xcex/8) that easily facilitates transducer fabrication using standard photolithography techniques, limits the SAW filter device to a 600 MHz operating frequency on ST-Quartz. The higher the sampling rate of the transducer for a given critical dimension (CD), the lower the operating frequency (fo), as provided by the equation foxe2x88x9dvo/(CD*S), where S is the number of electrode fingers per wavelength xcex(sampling rate of the transducer), vo is the SAW propagating velocity on the substrate, and the critical dimension (CD) is the smaller of either the electrode width or the gap width between adjacent electrodes.
A GSPUDT, like that disclosed in U.S. Pat. No. 5,073,763, can extend the operating frequency range of conventional DART SPUDT SAW filter devices to 1GHz on ST-Quartz due to a smaller sampling rate, e.g., 8 electrode fingers per 3xcex. However, although the sampling rate of the GSPUDT is small relative to the DART SPUDT it is still relatively high and thus limits the highest achievable operating frequency of the GSPUDT. Moreover, a multiple level metal deposition process is required at certain electrodes to create a predetermined, distributed internal reflection in both magnitude and phase necessary to provide unidirectionality.
U.S. Pat. No. 5,793,146 and the publication xe2x80x9cSingle-Phase Unidirectional Transducers Employing Uniform-Width Dithered Electrodesxe2x80x9d, by Wright et al., IEEE Ultrasonics Symposium, 1995, disclose a DSPUDT SAW filter device. The DSPUDT, like the DART SPUDT, has the advantage of single level metallization. However, DSPUDT techniques related to high frequency SAW filter applications include at least two disadvantages. First, the transducer reflection is a function of the metal thickness and the amount of electrode dithering. To generate a sufficiently strong transducer reflection for high performance low loss SPUDT SAW filters, either the electrode metal must be prohibitively thick, or certain electrode gaps must become very narrow to accommodate extensive dithering. The result of the prohibitively thick electrode finger metal is high insertion loss due to bulk wave conversion and distorted filter response due to mass loading. And by narrowing the electrode gaps, the critical geometry is forced below current fabrication thresholds which, as a result, limits the achievable highest operating frequency. Second, the reflection function implementation accuracy of a SAW device is determined by how accurately the dithered fingers (electrodes) are photolithographically printed on a substrate mask. And, because of the finite address size of the e-beam machine used to form the electrodes, the electrodes can only be dithered in a discrete fashion. Therefore, any misplacement of the electrodes due to grid snapping will interrupt the dithering pattern of the whole period resulting in large reflection function implementation error, especially at high operating frequencies where the critical geometries are small. Moreover, any misplacement of the least dithered electrodes in a period further magnifies implementation error.
As previously mentioned, various types of SPUDTs have been developed since the 1980""s and most of them rely on metal electrodes acting as acoustic wave reflectors. U.S. Pat. No. 4,353,046, on the other hand, integrates conventional acoustic wave reflector technology with grooved reflector structures so that distributed internal reflections, once considered detrimental to a SAW device, are deliberately integrated within the interdigital transducer to produce desirable effects. However, the SAW devices like those disclosed by U.S. Pat. No. 4,353,046 are limited to a split-electrode geometry with a four electrode per wavelength device topology that is functionally excluded at higher operating frequencies because of their relatively small gap and electrode widths. Specifically, the SAW device structures like those described in the ""046 Patent have sampling rates of xcex/4 or 4 electrodes per the length of one period (xcex) and, as previously described, this relatively high sampling rate limits the device""s operating frequency.
As communication and like system technologies advance, there is a need for making higher operating frequency and improved performance SPUDT filters beyond the standards currently attainable. Therefore, based on techniques known in the art for surface acoustic wave (SAW) single-phase unidirectional transducers (SPUDTs), a new family of single-phase unidirectional transducers (SPUDTs) having the capability to facilitate SPUDT filter operation at higher frequencies with improved performance is desirable.
The present invention provides a surface acoustic wave transducer having a transduction length measured in wavelengths (xcex). The surface acoustic wave transducer is fabricated on a piezoelectric substrate. The transducer includes a pattern of M uniform-width, uniformly spaced, single-level interdigitated electrodes per each Nxcex of the transduction length, with adjacent electrodes having a center-to-center spacing of Nxcex/M. Within one Nxcex of the transduction length, the surface acoustic wave transducer includes at least one first reflective structure selectively formed either in a gap located between two adjacent electrodes or under selected electrodes fingers for creating a first finite localized surface acoustic wave reflection function other than zero in both magnitude and phase. Within one other Nxcex of the transduction length, the surface acoustic wave transducer includes at least one second reflective structure selectively formed either in a gap located between two adjacent electrodes or under selected electrodes for creating a second finite localized surface acoustic wave reflection other than zero in both magnitude and phase. The positions and the relative reflective strengths of the reflective structures achieve a finite reflection function of the transducer equal to zero or other than zero in both magnitude and phase.
In another aspect of the present invention, using the principles of the present invention, a unidirectional transducer with a predetermined non-zero reflection is provided. And, by fabricating two such unidirectional transducers on a single piezoelectric substrate a novel filter is provided wherein the reflective structures of each unidirectional transducer are designed such that the surface acoustic wave (SAW) of each transducer propagates substantially towards the other.